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Interference is the interaction of two or more waves. Waves move along their direction of propagation characterized by crests and troughs. Wherever two or more waves, either from one source by different paths or from different sources, reach the same point in space at the same time, interference occurs.

When the waves arrive in-phase (the crests arrive together), constructive interference occurs. The combined crest is an enhanced version of the one from the individual wave. When they arrive out-of-phase (the crest from one wave and a trough from another), destructive interference cancels the wave motion. The energy of the wave is not lost; it moves to areas of constructive interference.

Interference occurs in sound waves, light waves, shock waves, radio, and x rays. Waves display crests and troughs like the wiggles along the length of a vibrating jump rope. We see interference when ripples from one part of the pond reach ripples from another part. In some places the combination makes a large wave; in other places the waves cancel and the water appears calm. Radio, visible light, x rays, and gamma rays are waves with crests and troughs in the alternating electro-magnetic field. Interference occurs in all of these waves. Interference of sound waves causes some regions of a concert hall to have special behavior. Where the multiple reflections of the concert sound interfere destructively, the sound is muffled and appears "dead." Where the reflections are enhanced by adding constructively, the sound appears brighter, or "live." Switching the polarity of the wires on a stereo speaker also can result in the sound appearing flat because of interference effects.

The most striking examples of interference occur in visible light. Interference of two or more light waves appears as bright and dark bands called "fringes." Interference of light waves was first described in 1801 by Thomas Young (1773-1829) when he presented information supporting the wave theory of light.

White light is a mixture of colors, each with a unique wavelength. When white light from the sun reflects off a surface covered with an oil film, such as that found in a parking lot, the thickness of the film causes a delay in the reflected beam. Light of some colors will travel a path through the film where it is delayed enough to get exactly out of phase with the light reflected off the surface of the film. These colors destructively interfere and disappear. Other colors reflecting off the surface exactly catch up to the light traveling through the film. They constructively interfere, appearing as attractive Interference patterns created by the waves from several fallen drops of water. Photograph by Martin Dohrn. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.

color swirls on the film. The various colors on soap bubbles as they float through the air are another example of thin film interference.

Modern technology makes use of interference in many ways. Active automobile mufflers electronically sense the sound wave in the exhaust system and artificially produce another wave out-of-phase that destructively interferes with the exhaust sound and cancels the noise. The oil film phenomenon is used for filtering light. Precise coatings on optical lenses in binoculars or cameras, astronaut's visors, or even sunglasses cause destructive interference and the elimination of certain unwanted colors or stray reflections.

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